[TEX]a^3-3a^2+5a-2019=0 \Leftrightarrow a^3-3a^2+3a-1+2a-2018=0 \Leftrightarrow (a-1)^3+2a=2018[/TEX]
[TEX]b^3-3b^2+5b+2013=0 \Leftrightarrow b^3-3b^2+3b-1+2b+2014=0 \Leftrightarrow (b-1)^3+2b=-2014[/TEX]
Cộng theo vế 2 pt:
[TEX](a-1)^3+2a+(b-1)^3+2b=2018-2014[/TEX]
[TEX]\Leftrightarrow (a-1+b-1)[(a-1)^2-(a-1)(b-1)+(b-1)^2]+2a+2b-4=0[/TEX]
[TEX]\Leftrightarrow (a+b-2)[(a-1)^2-(a-1)(b-1)+(b-1)^2]+2(a+b-2)=0[/TEX]
[TEX]\Leftrightarrow (a+b-2)[(a-1)^2-(a-1)(b-1)+(b-1)^2+2]=0[/TEX]
Dễ chứng minh [TEX](a-1)^2-(a-1)(b-1)+(b-1)^2+2 > 0 \Rightarrow a+b-2=0 \Rightarrow a+b=2[/TEX]